1. Field of the Invention
The invention relates to an induction stator for an electromagnetic retarder as well as an electromagnetic retarder equipped with such a stator.
More particularly, the invention relates to a stator for electromagnetic retarders that are mounted directly on a gear case of a motor vehicle transmission, such as the axle or the gearbox housing of a motor vehicle.
Still more particularly, the invention relates to a stator for an electromagnetic retarder intended for mounting on the gearbox and/or axle of a vehicle, comprising at least one electromagnetic coil and one asymmetric pole shoe on each radial transverse end face of each electromagnetic coil attached by means of a central axial screw.
2. Prior Art
A vehicle electromagnetic retarder can be used to assist a conventional (service) brake comprising brake pads that come together and press against at least one wheel hub disc in order to brake the vehicle. With the presence of endurance braking systems such as the electromagnetic retarder in parallel with conventional brakes, particularly for heavy vehicles such as trucks, reducing vehicle speed is achieved more safely, especially in long descent situations where premature brake pad wear is inevitable. As a commonly used endurance braking system, the electromagnetic retarder thus makes it possible to limit replacements of worn parts and reduce the cost and time of vehicle maintenance.
Generally, an electromagnetic retarder comprises at least one stator and at least one rotor. If the stator is directly connected to a vehicle transmission gear case, such as a gearbox housing or a transmission axle housing, and does not intersect a drive shaft in the retarder assembly, it is referred to as a “Focal” retarder (registered trademark). Otherwise, that is to say, with electromagnetic retarders intended to be placed on a drive shaft between an axle and a gearbox, said drive shaft is divided into two parts or portions by at least one retarder, and is referred to as an “Axial” retarder (registered trademark).
An electromagnetic retarder of the Focal type, described for example in document FR 2 577 357, comprises an annular induction stator with a first front rotor and a second so-called rear rotor on either side. The annular induction stator is composed of a ring with an even number of coils, with axes parallel to that of the retarder, supported by a generally annular-shaped transverse flange, each coil having a cylindrical pole core made of magnetic material that passes through said flange and a winding of electrical wire surrounding said supporting pole core to form a magnetic pole, said core being fixed, in the direction of an axis of said core, perpendicular to a plane of the stator, and ending in flared ends partially covering the coil while retaining it.
The flared ends of the cores are generally composed of plates, called pole shoes, which have a symmetrical contour relative to a radial plane passing through the axis of the device and through the axis of the corresponding core, and which are mainly intended to extend and guide the magnetic effect of the coil.
The pole shoes perform several functions:                they maintain a certain distance between the coils and the armature so as to limit the axial magnetic forces of attraction exerted on the armature;        they ensure a good distribution of the magnetic flux in the armature, and thus an efficient generation of eddy currents;        they also play a mechanical role of retention and protection of the coils and/or of the pole cores.        
In some cases, the shoe has a particular shape that extends it axially beyond the coil in order to optimize the passage of magnetic flux in order to increase the torques due to creation of eddy currents in the retarders in question.
FR 2 574 228 describes an asymmetric pole shoe, meaning the circumferential cross-section of each pole shoe is asymmetrical with respect to the radial plane passing through the axis of the retarder and through the axis of the corresponding core, in that a “leading horn” extends farther from said plane than a “trailing horn.” The “leading horn” and the “trailing horn” of a pole shoe correspond, respectively, to the “upstream” or “front” edge and the “downstream” or “rear” edge of said shoe with respect to the relative travel direction of the continuous armature annular element, meaning that as it travels past each shoe, each point of said element moves from the “leading horn” to the “trailing horn” of said shoe.
Because the pole shoes face the armature disc, an area that is larger in front of than behind the radial plane passing through the pole axis makes it possible to achieve higher braking torque due to lower magnetic saturation on the rear side of the shoes, as compared to the conventional configuration where the shoes are arranged symmetrically with respect to the aforesaid radial plane.
An electromagnetic retarder of the Focal type having asymmetric pole shoes is described, for example, in document FR 2863787, and illustrated in prior art FIGS. 1A, 1B, 1C and 2A, 2B, 2C, attached to this description and presented below, in which an eddy current device comprises, in the center, a stator 1 comprising induction coils 2, and two armature rotors 3, one on each coaxial side of the stator, and rotatable relative to the stator around an axis of rotation X.
Each coil is traversed by a cylindrical pole core made of magnetic material, integral with an annular radial plate 4 and a thicker annular radial flange 5, whose peripheral edge is axially folded back to give it greater rigidity, arranged at the two opposite ends of the coils, these two ends each being equipped with a pole shoe 6 of a larger cross-section, attached with, for example, a central axial screw 7, on the outside of the core of each corresponding coil and on the annular radial plate 4 and the annular radial flange 5.
Each rotor (not shown) comprises at least one armature disc located opposite the corresponding pole shoes 6 with an interposed air gap (not shown), wherein, in relation to the direction (F) in which the armature disk travels past the pole shoes 6, each pole shoe faces the armature disc, with a larger area in front (leading horn E) than in the rear (training horn S) of the radial plane passing through the axis of rotation X and through the axis of the corresponding coil.
The two sets of asymmetric pole shoes, one respectively on each radial transverse end face of the set of coils, are oriented in the same direction relative to the direction of travel (F). In fact, by association with the two corresponding armature rotors, this makes it possible to achieve higher braking torques due to lower magnetic saturation on the rear side of the shoes compared to the conventional configuration where the shoes are arranged symmetrically relative to the aforesaid radial plane.
The stator 1, composed of a sandwich-type assembly (shoes 6—plate 4—coils 2—flange 5—shoes), is arranged such that it is also sandwiched between the two rotors (front and rear) along with air gaps. The stator 1 is supported by said generally annular-shaped transverse flange 5, equipped at its outer periphery with an axial stiffening edge 8, and mounted cantilevered on the housing in question with an openwork bell-shaped frame 9 by means of stator attachment lugs 10 integral with the stator flange 5, attached by screws to four arms 11 of the frame 9.
The frame is a rigid part that may be made, for example, of ductile cast type iron and has stator attachment arms 11 that match with the stator attachment lugs 10. These stator attachment lugs are traversed by the aforesaid screws, whose heads are supported on the outer face of the stator attachment lugs in order to attach the stator 1 to the arms 11 of the frame 9, which is itself already attached to the main body of the housing so as to achieve the direct mounting of the electromagnetic retarder, in a known manner, on the gear case of a motor vehicle transmission, and thus cantilevered at the exit from the gearbox 12 or axle housing 13 of said vehicle by its rotors (not shown).
The attachment of the Focal-type magnetic retarder stator with asymmetric shoes on two faces, oriented as explained above, has major drawbacks in practice. This type of configuration requires the use of either a stator specifically designed for a gearbox or a stator specifically for an axle, because the magnetic circuit is guided by the identically oriented asymmetric shoes on each face (left and right) of the stator, depending on the direction in which the armature rotor travels past the asymmetric pole shoes as described above. This requires a single installation direction for a stator having a guided magnetic circuit and means of support and attachment, for example, the support flange and the attachment lugs also capable of such shoe guidance. For each type of assembly, gearbox or axle, there is thus a different design; the two are not interchangeable.
In addition, the manufacture and marketing of these two types of specific retarders require complicated logistical management and thus additional investment.
A particular object of this invention is to overcome these drawbacks.